The present invention relates to amplifiers and filters, and more particularly to a transconductance amplifier with a common mode output current control circuit for controlling the common mode level of the output signal.
A differential input (In+, Inxe2x88x92) to differential output (Out+, Outxe2x88x92) amplifying device typically implements a transfer function described by (Out+xe2x88x92Outxe2x88x92)=A (In+xe2x88x92Inxe2x88x92) where xe2x80x9cAxe2x80x9d is the gain (or attenuation) of the amplifying device. The difference between the two outputs is determined by the amplifier, but the average value of the outputs, (Out++Outxe2x88x92)/2, otherwise referred to as the common mode output, is not necessarily determined or controlled by many such differential amplifying devices.
It is desired to control the common mode output for a differential amplifying device. There are other characteristics that are desired for an amplifying device. For example, it is often desired to provide a certain level of immunity to noise and fluctuations on the power supply line. It is also desired to provide as much linearity as possible of the transfer function between the input and output signals. Of course, many other characteristics and qualities are desired as known to those having skill in the relevant art.
A differential transconductance amplifier with common mode output control according to an embodiment of the present invention includes a reference circuit that provides a reference signal, a current mirror input circuit, a differential current mirror, a summing junction, first and second feedback amplifiers, first and second feedback current mirrors, and a differential output circuit. The current mirror input circuit develops a differential input current in response to a differential input voltage, where the polarities of the differential input current have a constant sum. The constant sum of the polarities of the differential input current is based on the reference signal, such as being proportional thereto. The differential current mirror mirrors the differential input current into first and second high impedance nodes. The feedback amplifiers develop a differential feedback current through the summing junction which has a common mode current based on the reference signal. The first and second feedback current mirrors generate feedback current into the high impedance nodes from the summing junction in response to variations of summing junction voltage to maintain the common mode current. The differential output circuit develops a differential output current related to the voltage of the high impedance nodes and/or the differential input current mirrored into the high impedance nodes. Resistor/capacitor (RC) compensation circuits may be coupled to the high impedance nodes to compensate open loop gain and to compensate the common mode output current loop.
The differential transconductance amplifier exhibits many beneficial characteristics, including control of the common mode output signal, excellent immunity to noise and fluctuations on the power supply line, a control loop with wide bandwidth, a control loop that shares the same compensation method as the differential amplifier to save die area (e.g., when implemented on an integrated circuit (IC) or the like), a highly linear differential input voltage to differential output current transfer function when used as a feedback amplifier, easy adjustment of the desired common mode output current, and operation that is independent of the source voltages.
Each portion of the overall circuit may be implemented in any one of several suitable manners using suitable and complementary circuit components. The present invention is illustrated using metal oxide semiconductor (MOS) transistors include PMOS and NMOS transistors. The present invention, however, is not limited to the particular circuit or devices illustrated. For example, similar operation may be achieved using bipolar type devices, such as NPN and PNP bipolar junction transistors. A similar transconductance amplifier may also built with N-type transistors replacing the P-type transistors and P-type transistors replacing the N-type transistors, with the power supply or source signals (e.g. Vcc and Ground) exchanged. Many other specific circuit variations are possible and contemplated.
In a particular configuration, the reference circuit includes a bias resistor coupled to the source of a PMOS transistor having its gate and drain coupled together. A current reference, such as a current sink or the like coupled to the drain of the PMOS transistor, causes a corresponding reference voltage to develop at the gate of the PMOS transistor. The current mirror input circuit may include a pair of PMOS transistors having sources coupled to a source voltage via corresponding bias resistors. The PMOS transistors have gates coupled to the reference signal and drains coupled together at a common junction. Another pair of PMOS transistors each have a source coupled to the common junction, a gate receiving a polarity of the differential input voltage, and a drain that develops a respective one of first and second polarity signals of the differential input current. In this manner, the current mirror input circuit develops the differential input current based on the differential input voltage and the reference signal. In particular, the different polarities vary according to the differential input voltage but have a total current that is proportional to the reference signal. In one embodiment, the total current of the differential input voltage is equal to twice the level of a reference current.
The differential current mirror may include a first current mirror coupled to receive and mirror a first polarity the differential input current into the first high impedance node, and a second current mirror coupled to receive and mirror a second polarity the differential input current into the second high impedance node. In this manner, the differential input current is mirrored into the high impedance nodes used to develop the output current. In one embodiment, the first current mirror comprising first and second NMOS transistors having their gates coupled together and their sources coupled to the common source signal. The drain of the first NMOS transistor is coupled to the drain of the third PMOS transistor and where the drain of the second NMOS transistor is coupled to the first high impedance node. The second current mirror is configured in similar fashion using a pair of NMOS transistors for mirroring current into the second high impedance node.
The feedback amplifiers may also be implemented with NMOS transistors, each having a gate coupled to a respective one of the first and second high impedance nodes, a drain coupled to the summing junction and a source coupled to the common source signal via a bias resistor. The feedback current mirrors may each include a PMOS transistor having a gate receiving the reference signal, a source coupled to the summing junction, and a drain coupled to a respective one of the first and second high impedance nodes. The differential transconductance amplifier receives power from first and second power supply sources, such as VCC and GND signals. The summing junction may be coupled to a power supply voltage through one or more resistors. The PMOS transistors of the current mirror input circuit and the differential current mirror may all be matched PMOS transistors having their gates coupled to receive a reference voltage developed by the reference circuit.
An amplifying device with common mode output control according to an embodiment of the present invention includes an input circuit, a differential current mirror, a differential feedback current mirror amplifier, and a differential output circuit. The input circuit is responsive to a differential input signal and develops a differential input current having a common mode based on a reference signal. The differential current mirror mirrors the differential input current into a differential feedback node. The differential feedback current mirror amplifier develops a differential feedback current into the differential feedback node responsive to the differential input current and based on the reference signal to maintain a constant common mode current. The differential output circuit develops a differential output current based on voltage developed at the differential feedback node.
The differential feedback current mirror amplifier may include a summing junction, first and second feedback amplifiers and first and second feedback current mirrors. The first and second feedback amplifiers are coupled to the summing junction and to the differential feedback node and develops the differential feedback current through the summing junction to have a common mode current based on the reference signal. The first and second feedback current mirrors receive the reference signal and are coupled to the summing junction and to the high impedance nodes and generate feedback current into the differential feedback node from the summing junction in response to variations of summing junction voltage. The amplifying circuit may include a differential RC compensation circuit coupled to the differential feedback node.
A method of controlling the common mode output current of a differential amplifier includes generating a reference voltage, controlling a voltage generated differential input current so that a sum of first and second polarities of the input current is constant and based on the reference voltage, applying the first and second polarities of the input signal into first and second high impedance feedback nodes, generating first and second output currents through a summing junction based on corresponding voltages of the first and second feedback nodes, and generating first and second feedback currents through the summing junction and into the first and second feedback nodes, respectively, based on voltages of the summing junction and the reference voltage.
The method may further include generating first and second polarities of a differential output current signals based on corresponding voltages of the first and second feedback nodes. The method may also include compensating the differential amplifier with first and second RC filters at the first and second feedback nodes, respectively.
An integrated circuit for radio frequency (RF) communications may include a differential voltage source asserting a differential voltage, a mixer cell having a differential current input, and a low pass filter feedback amplifier. The low pass filter feedback amplifier is implemented in a similar manner as the differential transconductance amplifier or the amplifying device with common mode output control as previously described.